1. Field of the Invention
The field of art to which the invention pertains is hydrocarbon processing. In particular, this invention pertains to catalytic cracking processes in which the activity maintenance of the catalyst is improved so that a higher activity level or a longer catalyst life can be realized.
2. Description of the Prior Art
In the conversion of crude petroleum oil to usable gasoline product, one of the major processing steps is to catalytically crack the oil boiling in the range of about 400.degree. to 1000.degree. F. The catalyst employed in these cracking processes is usually a silica containing material, most commonly in combination with alumina. The catalyst may be an acid treated silica-alumina clay; an amorphous gel combination of silica-alumina, silica-zirconia, silica-alumina-zirconia, or silica-magnesia; or the silica-alumina zeolites which may be bound in a matrix of clay or amorphous gel material. The catalyst is most commonly used as a powder. In the more common fluid catalytic cracking process, liquid or partially vaporized hydrocarbon feed stock generally contacts hot freshly regenerated catalyst in the lower section of a riser reaction zone. The amount of regenerated catalyst employed is sufficient to supply the heat of vaporization of the oil feed, the endothermic heat of cracking, and the sensible heat required to carry out the cracking reaction at the desired operating temperature. The mixture of oil vapor and catalyst flows up through the high velocity riser, where substantial or all of the desired cracking to the desired lighter products and to coke deposited on the catalyst occurs. The hydrocarbon vapors after separation from the catalyst then flow to a main fractionator where they are separated into such typical fractions as a light gas and gasoline overhead, light cycle oil and heavy cycle oil sidestreams, and a bottom stream which contains fine catalyst which was not collected in the cyclone separator system. The bottom stream is settled to produce a decanted oil fraction and a slurry oil which is returned to either the inlet or outlet of the reaction zone to recover the entrained catalyst fines. The spent catalyst separated from the hydrocarbon vapors in the disengaging zone and cyclone separator system flows to a stripper where the countercurrent flow of stream removes absorbed and interstitial hydrocarbons from the catalyst. The stripped catalyst flows either directly as a dense phase to the regenerator or it may be transported by air to the regenerator. The catalyst in the regenerator is usually maintained as a dense fluid bed, although transport type regeneration or multiple bed type regeneration may be employed. In the regenerator, the catalyst contacts oxygen containing gas which burns the freshly deposited coke from the catalyst.
The amount of air fed to the regenerator is sufficient to burn all the coke deposited in the reaction section and to maintain the residual carbon on the regenerated catalyst at a low level. The freshly regenerated catalyst flows from the regenerator bed through a standpipe and controlled slide valve to the bottom section of the reaction zone where, as previously mentioned, it contacts the feed stock to be cracked.
In these cracking operations, the conditions in the cracking section are usually 900.degree. - 1000.degree. F temperature, 10 - 25 psig pressure and catalyst residence times less than 1 minute. The temperature and pressure in the stripper are similar to the reactor conditions, but the catalyst residence time is 0.5 - 2.0 minutes. In the regenerator the temperature is 1100 - 1400.degree. F, pressure 10 - 50 psig and catalyst residence time 5 - 15 minutes. The total time the catalyst spends in one cycle of cracking, stripping and regeneration is about 10 minutes. Experience has shown that whereas the fresh catalyst may have a surface area of 400 - 600 sq.m./gr., the catalyst inventory has an area of only 50 - 150 sq.m./gr. It is thus apparent that the catalyst being used for the cracking only has 10 - 25% the activity of the freshly prepared catalyst. It is therefore necessary to add a substantial amount of fresh catalyst each day to maintain the activity level of the catalyst in the unit.
The extensive deactivation of the cracking catalyst has been the subject of extensive inventigation. It has been shown that catalyst deactivation can occur from the deposition on the catalyst of such inorganic metals as sodium, which may be present, for example, as a contaminant in the spray water feed; or nickel and vanadium, which may be present as contaminants in the oil feed. However, even when careful control of these contaminants is practiced, severe catalyst deactivation still occurs. It has been concluded that this deactivation of the catalyst is primarily due to the influence of steam. It has been demonstrated that most of the catalysts are thermally stable in the presence of dry air at temperatures as high as 1500.degree. F. However, in the presence of steam at temperatures even as low as 900.degree. F, catalyst deactivation does occur.
The mechanism of the influence of steam on the stability of the cracking catalyst has not been elucidated to date. Catalyst formulation changes have improved the stability characteristics of the catalyst, but extensive deactivation is still experienced in commercial practice. Steps have been taken to minimize the use of steam, but the practice of using large amounts of steam is still being followed. Steam is used with the oil feed to aid in the atomization of the feed and contact with the regenerated catalyst. Steam is used in the stripper to remove absorbed and interstitial hydrocarbons from the spent catalyst before burning with air to remove deposited coke on the catalyst. Steam is also used for aeration in standpipes and for purging in vessels. Attempts have been made to minimize the use of steam, but the alternate processing methods have not been attractive. Also, the complete elimination of steam is not possible, since a significant amount is made in the regenerator as a result of the burning of the hydrogen containing coke from the catalyst.